As a technique of removing an echo, an echo suppressor for making a comparison between the level of a signal of a produced sound and the level of a signal of a collected sound and reducing the gain of the collected sound side when the level of the signal of the collected sound is low is known. An echo canceller for estimating a transfer function of an acoustic transmission space and removing a pseudo echo component from a signal of a collected sound is also known.
Since the echo suppressor lowers the gain of the collected sound side, an echo is not transmitted to the associated party, but double talk (a state in which both talking parties talk at the same time) cannot be performed. The echo canceller enables the double talk, but cannot completely remove the echo if the produced sound volume is too large.
Then, a technique of detecting whether or not the state is a double talk state in real time and using an echo canceller if the state is the double talk state and using the echo suppressor if the state is not the double talk state (For example, refer to Patent Documents 1 and 2).    Patent Document 1: JP-A-10-163931    Patent Document 2: JP-A-10-229354